Ultrasound driven aggregation and surface silanol modification in amorphous silica microspheres
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Ultrasound driven aggregation and surface silanol modification in amorphous silica microspheres Sivarajan Ramesh, Yuri Koltypin, and Aharon Gedanken Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel (Received 30 December 1996; accepted 2 May 1997)
Post formed, silica submicrospheres synthesized by Stober’s method have been subjected to a high intensity ultrasound radiation (20 kHz, 100 Wycm2 ) and their size, morphology, and surface silanol structure modified in situ. The processed silica powders have been characterized by a variety of techniques, such as powder x-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), BET nitrogen adsorption, and FT-IR spectroscopy. The silica microspheres formed through an irreversible sol-gel transition have been shown to aggregate by the condensation of interparticle silanols to larger particles under the influence of the shock waves emanating from an imploding cavity. The particle size as a function of sonication time passes through a maximum, suggesting the disintegration of the aggregates on longer exposure to ultrasound radiation. The sonication of dried silica microspheres in an inert dispersant decalin also led to the aggregation of microspheres to a lesser degree, suggesting the deactivation of surface silanols. Infrared spectroscopic investigations suggest a disruption of the hydrogen bonded network of surface silanols. The observed morphological changes have been discussed in terms of direct effect of cavitation on well-formed spheres rather than changes in growth mechanism and capture of primary particles.
I. INTRODUCTION
The effect of ultrasound radiation on the outcome and kinetics of chemical reactions seems to be well understood as of today.1–3 The chemical effects of ultrasound radiation have been identified as stemming from a phenomenon known as acoustic cavitation, viz., the formation, growth, and implosive collapse of bubbles in a liquid.3 Since the advent of cavitation a variety of chemical processes, such as catalyzed organic conversions,4,5 organo-metallic reactions,6 and polymerizations,7 have been carried out under the influence of ultrasound radiation. However, the exploitation of the cavitational phenomena in the synthesis and processing of metals, alloys, and ceramic materials is still in the infant stage. Suslick et al. for the first time synthesized nanophasic amorphous iron by the sonochemical decomposition of Fe(CO)5 ,8 followed by the synthesis of amorphous alloys.9 They also reported the interparticle collision and melting of hard metals, such as chromium and molybdenum, driven by turbulent flow and shock waves produced during an acoustic cavitation.10 By taking into account the particle size, velocity, heat capacity, and heat of fusion Doktycyz and Suslick estimated the temperature reached during interparticle collisions to be in the range of 2600 –3400 ±C for particles of the diameter of about 10 m. We have earlier reported the control of
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